The invention relates to the continuous application of a liquid coating to a substrate strip. The invention is applicable to processes generally, in which a substrate strip is coated with a coating composition, but was devised primarily for use in the continuous galvanising of steel strip (wherein the liquid coating is molten zinc) or the continuous coating of steel strip with other liquid coatings such as molten aluminium zinc alloys or polymeric paint compositions.
It is commonplace in such processes firstly to apply an over thick layer of liquid coating material to the strip at a coating station and then strip surplus material from the over thick layer to the required thickness for the finished coat. The reduction of the over thick layer is generally carried out using a gas jet stripping apparatus.
Gas jet stripping apparatus of the prior art includes two elongated nozzles disposed one on either side of the strip""s pass line, which direct sheetlike jets of gas against the respective sides of the thickly coated strip. The two nozzles extend transversely of the strip at right angles to the direction of strip travel. Each gas jet impinges normally or at a certain angle sometimes as large as 30xc2x0 to the strip, and splits into two gas streams flowing over the surface of the strip. One such stream flows in the direction of strip travel and the other flows in the opposite direction. Thus, one of the streams flow counter to the oncoming over thick layer and blows material from the layer back upon itself. The net effect is to prevent all but a thin layer of coating material in close adherence to the substrate strip from travelling with the strip past the nozzles.
For any particular installation, each nozzle is at least as long as the maximum width of strip that may be processed by the installation. Thus, whenever strips of lesser width are being processed, the nozzles extend beyond the edges of the strip. It follows that, beyond the edges of the strip, the end portions of the gas jets meet in opposition, producing a turbulent flow pattern adjacent to the strip edges.
A major problem of the traditional gas jet stripping apparatus described above was that the margins of the finished coating adjacent the longitudinal edges of the strip were somewhat thicker than the main part of the coating. This is known as edge build up and is thought to be due to two causes:
(I) The original application of the over thick layer of coating material applies coating material not only to the wide side faces of the strip but also to the narrow edge faces. Thus, there is a slightly greater amount of surplus material to be held back by the counter flowing gas stream at the strip edges than there is remote from the edges, and this leads to the slightly greater finished coating thickness near the edges.
(ii) The aforesaid turbulent flow pattern reduces the effectiveness of the counter flowing gas stream near the strip edges.
Therefore it has been previously proposed to modify the traditional gas jet stripping apparatus by the addition of a baffle to a plate adjacent each of the strip edges and disposed between the nozzles of the opposed gas jets. The courier plate extends from the vicinity of the strip edge to the maximum strip width and is disposed between the counter flowing gas stream beyond the strip edges.
Each such baffle is a rigid flanged edge having a first portion which extends longitudinally of the strip adjacent a longitudinal edge of the strip, and a second portion, being an extension of the first portion, which diverges away from the edge of the strip in the upstream direction of the strip. The first portion of the flanged edge is spaced a small distance from the longitudinal edge of the strip to allow an unhindered passage of the strip past the baffle.
The effect of the baffle is to contain the stripping counter flowing gas stream (and shield it from the above mentioned turbulence) until such time as the stripping gas stream reaches the second portion of the baffle. The stripping gas stream adjacent the baffle then tends to follow the diverging second portion of the baffle and thus sweeps across the edge of the strip. In so doing it carries coating material from the margin across the edge and discharges it from the strip as free droplets, so reducing the marginal coating thickness.
It has been found that such prior known gas flow control baffles, at least as applied to strips travelling vertically upwardly from a coating station, become less effective at slow strip speeds. This is because at low strip speeds there is less coating material to be dragged up by the moving strip, and the gas pressure supply to the nozzle has to be reduced to ensure that a sufficiently thick final coating is achieved. It is thought that this reduces the tendency of the gas flow to follow the diverging baffle portion with consequent reduction in the discharge from the margin of the over thick coating.
The object of the present invention is to alleviate the above deficiency.
Accordingly, the invention provides a gas jet stripping apparatus for reducing the thickness of the liquid coating on a moving strip including a pair of opposed gas jet stripping nozzles defining a stripping region, and a flow diversion device for positioning between said nozzles in said jet stripping region and adjacent said moving strip, said flow diversion device including a baffle having a first portion extending longitudinally of the strip adjacent the longitudinal edge of the strip, a second portion diverging away from the edge of the strip, and a gas supply duct communicating with the strip side of the baffle for providing gas to at least the second portion of the baffle.
It is thought that by supplying supplementary gas adjacent the baffle and directed over at least the strip side of the second portion of the baffle, less counter flowing gas stream at the strip edges and edge build up is greatly reduced.
In a preferred form of the invention, the gas is supplied to the strip side of the baffle in proximity to the junction of the first and second portions of the baffle having a velocity component in a direction which is counter to the direction of travel of the strip.
The baffle may be provided with at least one injection outlet and the gas from the at least one injection outlet is directed towards the strip at an angle within the range of +30xc2x0 to xe2x88x9230xc2x0 to the longitudinal edge of the strip, preferably +20xc2x0 to xe2x88x9220xc2x0 and most preferably within the range of +15xc2x0 to xe2x88x925xc2x0, a positive sign denoting injection toward the strip edge (in the direction of the strip center) and a negative sign indicating gas injection in a direction away from the strip.
The gas supply duct may supply gas to a gas plenum or a plurality of plenum which in turn supply a plurality of injection outlets on the strip side of the edge baffle.
In another aspect of the invention there is provided a flow diversion for a jet gas stripping apparatus, said device including a baffle having a first portion extending longitudinally of the strip adjacent the longitudinal edge of the moving strip, a second portion diverging away from the edge of the strip, and a gas supply duct communicating with the strip side of the baffle for providing gas to at least the second portion of the strip side of the baffle.
In a further aspect of the invention there is provided a method of reducing the edge build up in the coating process of a moving strip.